1. Field of the Invention
The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a method for producing the same, and a non-aqueous electrolyte secondary battery using the positive electrode active material.
2. Description of the Related Art
As a main power supply and a backup power supply of miniaturized electronic equipments, a secondary battery having a high energy density is required. A lithium ion secondary battery is preferably used in applications described above because it has a high voltage and a high energy density. In particular, advances have been made in research and development of a high energy density lithium ion secondary battery using a lithium nickel composite oxide such as LiNiO2 as a positive electrode active material.
Solid State Ionics Vol. 80 (1995) (Non-Patent Document 1) discloses a lithium nickel composite oxide Li1−xNi1+xO2 obtained by wet-mixing LiOH.H2O and Ni(NO3)2.6H2O and sintering the mixture in air. In case of Li1−xNi1+xO2, an amount x of nickel excessively exists at the occupying position of lithium. It is reported that the discharge capacity increases as the value of x decreases. Also, it is disclosed that a secondary battery using Li1−xNi1+xO2 as the positive electrode active material has a discharge capacity of 220 mAh/g upon first discharging based on a Li/Li+ electrode when charged or discharged within a range from 3.0 to 4.5 V. It is also disclosed that the charge and discharge capacity of the secondary battery decreases to 200 mAh/g only by repeating charging and discharging about 10 times.
Also, Japanese Unexamined Patent Publication (Kokai) No. 7-105950 (Patent Document 1) discloses that it has hitherto been difficult to obtain a secondary battery having a high discharge capacity with good reproducibility when LiNiO2 is used. It is also discloses that a secondary battery having a high discharge capacity can be obtained with good reproducibility by using a positive electrode active material made of LiNiO2 particles in which primary particles capable of forming aggregated particles like secondary particles have a particle size of 1 μm or less. The patent document discloses, as a method for producing the positive electrode active material, a method of mixing lithium carbonate with nickel oxide and heat-treating the mixture.
In both lithium nickel composite oxides described in Non-Patent Document 1 and Patent Document 1, nickel exists at the occupying position of lithium. Therefore, there is a problem that the charge and discharge capacity decreases because of excess nickel.
As the positive electrode active material obtained as a result of a solution of the above problem, for example, LiNi0.5Mn0.5O2 obtained by the method disclosed in “Science Vol. 311 (2006) page 977” (Non-Patent Document 2) and “Electrodes with High Power and High Capacity for Rechargeable Lithium Batteries, Kisuk Kang and four others, “On line Science Homepage, Internet URL: http://www.sciencemag.org/cgi/content/full/311/5763/977/DC1 searched on Sep. 11, 2006” (Non-Patent Document 3) is known. The method is as follows.
First, NaNi0.5Mn0.5O2 is synthesized by wet grinding and mixing Na2CO3, Ni(OH)2 and Mn2O3 in a ball mill for one day and reacting the mixture in air at 900° C. for 24 hours. Then, NaNi0.5Mn0.5O2 is reacted in a molten salt composed of a 10-fold amount of LiNO3 and LiCl, thereby ion-exchanging sodium in NaNi0.5Mn0.5O2 with lithium to obtain LiNi0.5Mn0.5O2.
In LiNi0.5Mn0.5O2 obtained by the above method, since exchange of the occupying position of lithium with that of nickel is suppressed, a decrease in the charge and discharge capacity caused by the presence of excess nickel at the occupying position of lithium can be suppressed. It is also discloses that a secondary battery using the resulting LiNi0.5Mn0.5O2 as a positive electrode active material attains a capacity of more than 200 mAh/g based on a Li/Li+ electrode when charged and discharged within a range from 3.0 to 4.6 V.
However, particles of the positive electrode active material LiNi0.5Mn0.5O2 described in the aforementioned Non-Patent Documents 2 and 3 have a problem that the capacity remarkably decreases with an increase of the number of charging and discharging cycles. Specifically, it is described that the capacity decreases to about 80% of the initial capacity after 30 charging and discharging cycles.